1. Field of the Invention
The present invention relates to a power semiconductor module that may be applied to a converter or inverter of a power switching device, for example.
2. Description of the Related Art
First, the assembled structure of a conventional example of such a power semiconductor module (in which six elements form a structure for three circuits), will now be described using FIGS. 9a and 9b. In the figures, a metal base (copper base plate) 1 is provided for radiating heat, which is a heat sink, an outer case 2 is provided having terminals formed integrally, and a main circuit terminal 3 is provided in this outer enclosing case 2. A control terminal 4, a power circuit block 5, a control circuit block 6, and bonding wires 7 including internal wiring are also provided.
FIGS. 9a and 9b illustrate the power circuit block 5 having six of each of a power semiconductor chip 9, such as an IGBT, and a free-wheeling diode 10, mounted on a ceramic substrate 8 that is in the shape of a rectangular oblong. Further, as shown by the schematic view of FIG. 4, the ceramic substrate 8 is a direct copper bonding substrate including an upper circuit plate 8b, and a lower plate 8c, which are plated with copper foil and are directly joined to the upper surface and lower surface, respectively, of a ceramic plate 8a made from alumina, aluminum nitride or silicon nitride. A semiconductor pattern is formed on the upper circuit plate 8b. The semiconductor chips 9 are mounted on the upper circuit plate 8b via a solder join layer 11 including solder. Referring to FIGS. 9a and 9b, the control circuit 6 is made such that drive ICs 6b, which are for driving power circuit elements, and circuit components associated with these drive ICs, are mounted on a U-shaped printed substrate 6a surrounding the power circuit block 5.
In addition, the power circuit block 5 and the control circuit block 6 are placed side by side on a metal base 1. Thereupon, the lower plate 8c of the ceramic substrate 8, on the power circuit block 5, and the metal base 1 are joined using Sn—Pb solder (this solder is represented by “12”) to conduct heat generated by the power semiconductor chips 9 to the metal base 1 via the ceramic substrate 8. Further, the printed substrate 6a of the control circuit block 6 is affixed to the metal base 1 using an adhesive.
The outer case 2 having terminals integrally formed therein is joined to the metal base 1 using an adhesive. Specifically, the outer case 2 is placed over the power circuit block 5 and the control circuit block 6 is mounted on the metal base 1 so as to enclose the power circuit block 5 and the control circuit block 6. The inner lead portions 3a of the main circuit terminals 3 are soldered to the printed pattern of the ceramic substrate 8, and control terminals 4 become internal wiring as a result of bonding wires 7 to the printed pattern of the printed substrate 6. Further, after a gel-like filler material (silicon gel, for example), has been injected into the outer enclosing case 2 to seal off each of the circuit blocks using resin, the outer enclosing case is covered with a lid (not shown) to thus complete the power semiconductor module.
Power semiconductor modules like those mentioned above are extensively employed in various fields, from low-capacity devices for everyday use to high-capacity devices which are used industrially or in vehicles, for example. In such cases, low-capacity modules for everyday-use, for which the degree of reliability required is not particularly high, and in which the amount of heat generated by power semiconductor elements is limited, can be designed and manufactured with barely any restrictions being placed on the material and size of each of the components. On the other hand, where high-capacity modules are concerned, which are typically used in power circuits of vehicle drive devices, there is sometimes an increased amount of heat generated per unit area of power chips in the high-capacity modules. Because the size of such power chips is smaller and more compact, there is a demand for high reliability and long life in high-capacity modules compared to the low-capacity modules in products manufactured for everyday use.
For example, a heat cycle test to which manufactured power modules are subjected to include conditions for one heat cycle: −40° C. (60 minutes), followed by room temperature (30 minutes), followed by 125° C. (60 minutes), and followed by room temperature (30 minutes). In contrast, ordinary general-purpose products are subjected to a test of around 100 cycles. Accordingly, the reliability required for products manufactured for vehicle drive devices is such that such products must sufficiently withstand a heat cycle test of 3000 cycles.
Satisfying these requirements of high reliability and long life naturally means improving the reliability of the power semiconductor chips themselves. In addition, a pressing problem regarding these power semiconductor chips includes ensuring a durability that can sufficiently withstand high levels of heat radiation, and the severe heat cycles accompanying the motion of the vehicle, at the same time as ensuring the required electrical insulation resistance.
In a structure in which a ceramic substrate 8 mounted on a metal base 1 is joined thereto with solder, as shown in FIG. 4, the problems set forth above are accompanied by the problems noted hereinbelow.
(1) Thermal stress attributable to a difference in the thermal expansion of the ceramic substrate/metal base: specifically, while a rate of thermal expansion of the ceramic substrate is 7 ppm/K for alumina, 4.5 ppm/K for aluminum nitride, and 3 ppm/K for silicon nitride, a rate of thermal expansion of the metal base (copper base) is 16.5 ppm/K, which provides for a large difference between the thermal expansion rates of the ceramic substrate and the metal base. Meanwhile, the yield strength of the solder (Sn—Pb solder), which forms the join between the metal base and the ceramic substrate, is low at around 35 to 40 MPa, and, when the stress at the solder join portion repeatedly increases with successive heat cycles on account of the difference between the thermal expansion rates of the metal base and the ceramic substrate, this solder eventually burns out.
In addition, with regard to a life of the solder until burn-out, after performing various testing on the life of the solder in addition to a corresponding analysis, it has been established that there was a tendency for the life of the solder to be heavily dependent on the conditions below. In other words, assuming that the ceramic material of the ceramic substrate soldered onto the metal base is the same: (a) an increase in the plate thickness of the ceramic substrate entails an increase in the strain generated in the solder join portion, and the life before burn-out becomes short (dependence on the thickness of the substrate); (b) an increase in the surface area of the ceramic substrate entails an increase in the strain generated in the solder join portion, and the life before burn-out becomes short (dependence on the surface area of the substrate); (c) an increase in the ratio of the length and breadth of the ceramic substrate entails an increase in the strain generated in the solder join portion, and the life before burn-out becomes short (dependence on the shape of the substrate); and (d) a decrease in the thickness of the solder constituting the solder join layer entails an increase in the strain generated, and the life before burn-out becomes short (dependence on the thickness of the solder).
(2) Thermal conductivity between the ceramic substrate/metal base: The heat generated by the power semiconductor chip is thermally conducted via the ceramic substrate to the metal base, which functions as a heat sink, and is radiated from the metal base to the outside. Consequently, it is necessary to keep the resistance to thermal conduction as low as possible between the ceramic substrate and the metal base.
Accordingly, (a) while a thermal conduction rate of the ceramic substrate is 20 W/mK for alumina, 180 to 200 W/mK for aluminum nitride, 70 to 100 W/mK for silicon nitride, and the thermal conduction rate of the copper base is 398 W/mK, the thermal conduction rate of the solder (Sn—Pb solder), which forms a join between the metal base and the ceramic substrate, is 40 to 50 W/mK. As a result, a thermal resistance of the solder join portion has a major influence on the thermal conduction in the thermal conduction path between the power semiconductor chip and the metal base. (b) Further, upon soldering the ceramic substrate onto the metal base, when voids (air bubbles) are generated in the solder join layer, these voids provide for a thermal resistance and therefore obstruct the radiation of heat. Consequently, an increase in the thickness of the solder layer of the solder join layer entails lower heat radiation, which has an adverse effect on the reliability and durability of the module.